Wilson Teixeira

A review of the geochronology of the Amazonian Craton: Tectonic implications

Abstract The available geochronological data for the Amazonian Craton permit delineation of its main age provinces and their respective tectonic development. Demarcation of the boundaries of each age province is primarily based on Rb-Sr whole-rock isochrons, supported by K-Ar determinations on minerals. Some U-Pb results on zircons, Pb-Pb whole-rock isochrons, and Sm-Nd model ages complement these data in key areas. At the center of the Amazonian Craton lies the Archean age, Central Amazonian Province bounded in its northern part by the Maroni-Itacaiunas mobile belt, which developed during early Proterozoic times. The western and southern margins of the Archean core are bordered by three tectonic provinces: (1) the Rio Negro-Juruena magmatic arc, (2) the Rondonian mobile belt, and (3) the Sunsas mobile belt. Each belt borders the western margin of the preceding belt, and each records a successively younger middle Proterozoic orogeny. Within the Central Amazonian Province, −2.7 Ga age determinations have been obtained from the Serra dos Carajas area. Although the Maroni-Itacaiunas belt includes several reworked Archean nuclei, rock units formed during the 2.2-1.9 Ga Trans-Amazonian Orogeny predominate. The isotopic evidence suggests that a mantle-derived component constituted a significant portion of the crust accreted at that time. The Rio Negro-Juruena Province is of Mid-Proterozoic age (1.75-1.50 Ga), and includes almost entirely mantle-derived material, juxtaposed with the pre-existent continental mass along a series of successive magmatic arcs. The Rondonian and Sunsas provinces both exhibit older sialic basements (>1.5 Ga). The former is characterized by important tectonomagmatic events in the 1.45-1.25 Ga interval, whereas the latter records tectonic reworking at 1.1-0.9 Ga. Finally, the Amazonian Craton is bordered to the east by the late Proterozoic Paraguai-Araguaia (or Araguaia-Tocantins) fold belt.

Proterozoic geologic evolution of the SW part of the Amazonian Craton in Mato Grosso state, Brazil

Abstract This paper presents new geochronologic, isotopic, and geochemical data bearing on the evolution of Proterozoic crust in SW Mato Grosso state, Brazil, which is at the southern end of the ≈1.6–1.8 Ga Rio Negro–Juruena orogenic belt of the Amazonian Craton (Amazonia). Our data define three major crustal events: (i) the Alto Jauru terrane occurs in the eastern part of the region and is comprised of island arc-related rocks with U/Pb ages from 1.79 to 1.74 Ga. These rocks have e Nd ( t ) values range from +2.8 to +2.0 with crustal residence ages ( T DM ) from 1.93 to 1.78 Ga, indicating a mainly juvenile signature at 1.8 Ga; (ii) the Cachoeirinha suite occurs in the central part of the region and consists of calc-alkaline plutons emplaced into Alto Jauru terrane host rocks. These plutons have U/Pb ages from 1.56 to 1.54 Ga and e Nd ( t ) values ranging from +1.0 to −0.8, with crustal residence ages ( T DM ) from 1.88 to 1.75 Ga. We interpret this orogen as the roots of a continental margin arc built upon basement comprised of the Alto Jauru terrane; (iii) the Santa Helena batholith occurs in the western part of the region and is a large, elongate body of calc-alkaline rocks ranging from granodiorite to highly evolved granite. These units yield U/Pb ages from 1.45 Ma to 1.42 Ga, with e Nd ( t ) values ranging from +4.1 to +2.6 and crustal residence ages ( T DM ) from 1.70 to 1.50 Ga. The Rio Alegre domain occurs west of the Santa Helena batholith and includes juvenile 1.52 to 1.47 Ga volcanic and mafic plutonic rocks. Regional geologic relationships suggest that these rocks are part of the crust into which the Santa Helena batholith was emplaced. We interpret this batholith as the magmatic core of a juvenile arc accreted to the edge of the Alto Jauru terrane, with incorporation of some older crust (Alto Jauru terrane) in the east and derivation from mainly juvenile crust (Rio Alegre domain) in the west. The Rio Branco suite occurs to the east of exposed Alto Jauru terrane rocks as large hills protruding through younger Aquapei Group sedimentary rocks. It consists of gabbro and granophyric rocks with U/Pb ages of 1.47 Ga (gabbro) and 1.43 Ga (granophyre) and inherited older Nd, suggesting hinterland derivation from Alto Jauru terrane basement during development of the Santa Helena batholith. Several undeformed 1.5–1.4 Ga granitic plutons occur within the Alto Jauru terrane; these are also regarded as inboard manifestations of subduction related magmatism associated with accretion of the Rio Alegre domain and formation of the Santa Helena batholith. The siliciclastic Aguapei Group was deposited sometime between 1.0 and 1.4 Ga; it overlies Jauru terrane basement, rocks of the Santa. Helena batholith, and the Rio Alegre domain. In the east, it is flat-lying and undeformed, but in the west it is deformed and metamorphosed in the NNW trending Aguapei thrust belt. Farther west the Aguapei Group is horizontal, undeformed, and overlies the Paleoproterozoic Paragua block in Bolivia, where it has been correlated with the Sunsas Group. The Aguapei thrust belt has K/Ar cooling ages of about 930 Ma and is apparently a foreland fold and thrust belt formed by reactivation of an older rift basin during the 1.0 Ga Sunsas orogeny, which occurs west of the Paragua block in Bolivia.

An outline of Early Proterozoic crustal evolution in the São Francisco craton, Brazil: a review

Abstract The Early Proterozoic evolution of the Sao Francisco craton (SFC) occurred between 1.8–2.5 Ga and includes the Transamazonian cycle (1.9–2.2 Ga), one of the main periods of crustal growth affecting the South American continent. This synthesis of the available data is largely concerned with the Early Proterozoic terranes of the SFC and presents a modeled crustal evolution integrated with that of the West Congo craton. Interpretation of radiometric ages, geochemical and structural data, metamorphism (P, T conditions), and lithostratigraphical correlations leads to the characterization of three Early Proterozoic belts in the SFC, named: the Correntina-Guanambi, Itabuna and Mineiro belts. These belts are here described following a geographic scenario as given by their representatives (medium- to high-grade sequences, low-grade supracrustal sequences and intrusive igneous rocks). The anatomy and regional structures of these belts indicate that they developed along the margin of a large Archean protocontinent, although characteristic older mantled gneiss domes are also found within the Itabuna and Mineiro belts. Available isotopic evidence suggests an episodic growth of the continental crust during the Late Archean (2.6–3.2 Ga) with the formation of microcontinents that gradually coalesced around 2.6 Ga ago. After the Archean-Proterozoic transition, the tectonic evolution of the Proterozoic mobile belts evolved by mantle accretion/differentiation and crustal reworking. The main Early Proterozoic magmatic arc of the SFC is represented by the Itabuna mobile belt, whereas the Minciro belt is chiefly ensialic. The nature of the Salvador-Correntina belt is still poorly constrained by geochronological and geochemical data. By the end of the Transamazonian cycle, around 1.8–1.9 Ga ago, the SFC was fully cratonized.

40Ar/39Ar dating of 1.0–1.1 Ga magnetizations from the Sa˜o Francisco and Kalahari cratons: tectonic implications for Pan-African and Brasiliano mobile belts

Paleomagnetic poles from 1.1-1.0 Ga dyke swarms in eastern Brazil (Sa˜o Francisco Craton) are compared with 1.0 Ga poles from granulites of the Namaqua Province in southern Africa (Kalahari Graton). The intrusive ages of dykes are estimated from40Ar/39Ar dating of outgassed biotites from baked country rocks. The age of magnetization for the granulites is derived by combining40Ar/39Ar hornblende and biotite dates. When restored to a Mesozoic pre-drift configuration the paleomagnetic poles are in crude spatial agreement but are temporally discordant. To satisfy both paleomagnetic and geochronologic constraints, a reconstruction involving separation of the Sa˜o Francisco and Kalahari cratons is required, indicating that the intervening Pan-African (Brasiliano) mobile belt may record a craton-craton collision.

Pb, Sr and Nd isotope constraints on the Archaean evolution of gneissic-granitoid complexes in the southern São Francisco Craton, Brazil

Abstract Three Archaean gneissic complexes (Bonfim, Belo Horizonte and Campo Belo) in the southern part of the Sao Francisco craton were studied by means of UPb (zircon and titanite, monazite) and SmNd, RbSr and PbPb (whole rock) methods. In the Bonfim metamorphic complex the period 2780-2700 Ma is marked by in situ anatexis of gneisses, intrusion of tonalites, mafic dikes and late granites, and deposition of the Rio das Velhas greenstone belt. SmNd T DM model ages on its gneisses, amphibolites and granites range from 2800 to 3000 Ma, indicating involvement of older sialic crust in the 2780-2700 Ma event. In the Belo Horizonte metamorphic complex UPb zircon geochronology has yielded ages of 2860, 2776 and 2712 Ma and RbSr whole rock isochrons on its gneisses and migmatites yielded ages in the range 2800-2750 Ma (with large uncertainties) and 87 Sr/ 86 Sr initial ratios of 0.700-0.710. The highest initial ratio (0.710) obtained for the migmatites suggests that ensialic episodes participated in the tectonomagmatic evolution. Existence of older crust is suggested by UPb zircon ages of ∼ 3030, 2920 and 2880 Ma from inherited grains in metavolcanics and orthogneisses. RbSr ages of 2250-2130 Ma on granitoids and UPb (titanite, monazite) ages from 2320 to 2030 Ma on granitoids, felsic veins and amphibolite enclaves indicate reworking of this complex in the Palaeoproterozoic. In the Campo Belo metamorphic complex granite and greenstone lithologies have yielded ages of ∼ 3380-3000, 2900 and 2650 Ma by UPb zircon and RbSr and PbPb whole rock methods. T DM ages on its granites, gneisses and granulites have yielded ages from 3070 to 2780 Ma indicating involvement of older sialic material during the 2650 Ma event. These complexes indicate major events during the period 2860-2700 Ma, including reworking of established sialic crust with components up to 3380 Ma old. Late granites were injected between 2700-2600 Ma, preceding final assembly and stabilization of the Archaean crust.

Restored transect across the exhumed Grenville orogen of Laurentia and Amazonia, with implications for crustal architecture

New 40 Ar/ 39 Ar analyses from a transect across the major tectonic units of the southwest Amazon craton document the heterogeneous effects of the late Mesoproterozoic collision with the Grenville margin of North America. Basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized iso- topic age resetting at 1.18-1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement. In the Nova Brasilandia belt between these two cratons, new 40 Ar/ 39 Ar data record cooling through 920 Ma after the granulite facies deformation of this suture zone. Regional cooling rates calculated from compiled U/Pb, 40 Ar/ 39 Ar, and Rb/Sr thermochronologic data are used to establish post- Grenvillian exhumation patterns for the southwest Amazon and the North American belt. Paleodepths calculated for 1.0 Ga along a transect of the restored 1300-km-wide belt vary from uniformly deep levels (15-30 km) exposed in North America to shallower levels (5- 15 km) observed in the southwest Amazon. We interpret this difference as reflective of a change in tectonic architecture, i.e., thrust-dominated deformation in Laurentia versus strike-slip dominated deformation in the Amazon, with a commensurate variation in crust- al thickness. This interpretation explains the widespread preservation of both pre- Grenvillian ages and collisional ages from the Amazon craton, in contrast with the more homogeneous array of cooling ages from the North American Grenville Province marking the postorogenic extensional collapse of an overthickened crust. The asymmetrical oro- genic architecture from the reconstructed Grenville belt mirrors cross sections proposed for modern orogenic belts where deep-crustal rocks are not yet exposed.

40Ar/39Ar geochronology at the Instituto de Geociências, USP: instrumentation, analytical procedures, and calibration

Laser heating Ar-40/Ar-39 geochronology provides high analytical precision and accuracy, mum-scale spatial resolution. and statistically significant data sets for the study of geological and planetary processes, A newly commissioned Ar-40/Ar-39 laboratory at CPGeo/USP, Sao Paulo, Brazil, equips the Brazilian scientific community with a new powerful tool applicable to the study of geological and cosmochemical processes. Detailed information about laboratory layout, environmental conditions, and instrumentation provides the necessary parameters for the evaluation of the CPGeo/USp Ar-40/Ar-39 suitability to a diverse range of applications. Details about analytical procedures, including mineral separation, irradiation at the IPEN/CNEN reactor at USP, and mass spectrometric analysis enable potential researchers to design the necessary sampling and sample preparation program suitable to the objectives of their study. Finally, the results of calibration tests using Ca and K salts and glasses, international mineral standards, and in-house mineral standards show that the accuracy and precision obtained at the Ar-40/Ar-39 laboratory at CPGeo/USP are comparable to results obtained in the most respected laboratories internationally. The extensive calibration and standardization procedures under-taken ensure that the results of analytical studies carried out in our laboratories will gain immediate international credibility, enabling Brazilian students and scientists to conduct forefront research in earth and planetary sciences.

Isotopic signatures of Paleoproterozoic granitoids from the southern Sao Francisco Craton and implications for the evolution of the Transamazonian Orogeny

Abstract The southern Sao Francisco Craton, northeastern Brazil, consists of an Archean block surrounded by a Paleoproterozoic belt related to the Transamazonian Orogeny (ca. 2.0 Ga). A calc-alkaline plutonic arc developed within the belt and the granitoid plutons comprise two distinct groups. One group displays Archean T DM ages (3.07–2.62 Ga), e Nd( t ) values between −11.0 and −3.8 and high initial 87 Sr/ 86 Sr values, and it consists mainly of peraluminous granites. T DM ages for the other group are Paleoproterozoic (2.43–2.27 Ga), and e Nd( t ) values range between −2.8 and +1.3; the plutons are metaluminous tonalites (trondhjemites) to granodiorites. The Transamazonian granitoids can be related to contrasting source-regions, from mantle- to crust-derived ones. A number of them are probably derived from mixing of Paleoproterozoic juvenile material and variable proportions of Archean crust material. Magmatism related to deep faulting, during the compressional stages of the Transamazonian Orogeny, is a plausible model for granitoid generation. The contribution of mantle-derived material to the granitoid sources supports the idea that a significant episode of new crust formation occurred during the Transamazonian Orogeny.

Paleomagnetism of Middle Proterozoic (1.01 to 1.08 Ga) mafic dykes in southeastern Bahia State—São Francisco Craton, Brazil

Paleomagnetic results from profuse Middle-Late Proterozoic mafic dyke swarms in southeastern Bahia State (Sa˜o Francisco Craton) yield either an easterly direction with high upward inclination or a westerly direction with high downward inclination isolated during AF and/or thermal treatments. Thermal demagnetization behavior and thermochronologic and petrologic considerations indicate that these remanent components originated as primary TRMs. Four groups of directions were distinguished from dykes in spatially distinct areas: Ilheus normal polarity (D = 60.0°; I = −68.8°; α95 = 2.6°; N = 17) , Olivenca normal polarity (D = 82.4°; I = −71.0°; α95 = 5.1; N = 31), Itajudo Coloˆnia (D = 99.0; I = −71.9 ; α95 = 5.9°; N = 23) and Olivenca reversed polarity (D = 298.8°; I = 60.7°; α95 = 6.4°; N = 18), which yield paleomagnetic poles located at 100.4°E; 30.3°N (IN), 107.0°E; 16.1°N (ON), 111.0°E; 7.7°N (IC) and 280.2°E; 17.0°N (OR), respectively. These poles define an APW path for the Sa˜o Francisco Craton between the time interval 1.01–1.08 Ga which is characterized by at least two polarity intervals.

Polyphase Archean evolution in the Campo Belo metamorphic complex, Southern São Francisco Craton, Brazil: SHRIMP U-Pb zircon evidence

Abstract Reconnaissance SHRIMP U-Pb geochronology on three zircon populations from a migmatite of the Campo Belo Complex reveals a polyphase Archean history. Group 1 zircons (three grains) yielded a weighted mean 207 Pb/ 206 Pb age of 3,205±17 Ma, interpreted to be a relict age component within the migmatite. This age compares well with most of the T DM ages available for the Archean metamorphic complexes in the Southern Sao Francisco Craton and indicates the first emergence of continental crust. Group 2 zircons (three grains) yielded a weighted mean 207 Pb/ 206 Pb age of 3,047±25 Ma, interpreted to be a second tectonic event in the area. This event is compatible with the formation of medium to high-grade gneisses with e Nd (t) parameters close to zero and mantle-like single stage Pb signature ( 238 U/ 204 Pb=8.18). Group 3 zircons (three grains) yielded a weighted mean 207 Pb/ 206 Pb age of 2,839±17 Ma, which is considered to be the crystallization age of the neosome—the third tectonic event in the CBC. The fact that the migmatite sample contains three generations of melts is coherent with the observed variation of the e Nd(2.84 Ga) values of the CBC gneissic rocks between +0.9 and −3.0. This indicates that reworking processes have had an important role during the crustal evolution. The Neoarchean final evolution of the area is associated tectonically with the Rio das Velhas orogeny (2,780–2,700 Ma), during which migmatitic rocks were formed 2,690±56 Ma ago, in the southern edge of the Campo Belo Complex. In this region, mafic-ultramafic intrusions emplaced at ca. 2.75 and 2.66 Ga ago are associated with major distensional phases of this orogeny. Finally, a low-temperature episode took place at ca. 2,650 Ma ago within the Complex, as evidenced by resetting of the Rb–Sr and Pb–Pb whole rock isotopic systematics of granitoid rocks long after their primary crystallization.